1. Field of the Invention
The present invention relates to the detection of turbulence in fluids, and is particularly applicable to determining whether a hot body viewed by a thermal detector array is a flame.
2. Description of the Related Art
In flame detection, it is useful to be able to distinguish flame signals (the majority of which are dominated by low frequency modulation) from other low frequency events (false alarms). These false alarms generally emanate from some stationary hot black body (e.g.: the sun, electric fire) which are modulated by objects that move between them and the detector, creating modulated signals that can look very like flames.
In contrast, fires create turbulence when the burning fuel creates balls of hot gas. So, given that the fire is positioned such that it covers more than one pixel on the array, this turbulence will manifest itself in the image. In particular, this work focuses on the differences and similarities between a pixel's time-series data and those of its horizontal and vertical neighbors. The novelty of this invention lies in the use of statistical measurement of variation as a pattern recognition procedure.
GB-A-2269454 discloses a method of flame detection by imaging. It is a tenet of this method that an image of a flame will have a structure such that its measurement over time will identify the various regions of the flame. Cross-correlation techniques are used, but these are standard statistical measures used in numerous applications.
Our earlier patent application GB-A-2366369 (Upwards Correlation) discloses a method of identifying turbulence in fluids based on the progression of flame features in a single direction within a flame cluster. Thus this method needs to track over time and assumes some underlying structure in the flame cluster, whereas the new invention always works at zero lag and makes no assumptions about spatial organization.
A difficulty of Upwards Correlation in practice occurs when the speed of thermal bleed (described below) matches the speed of flame movement. Under these circumstances, it becomes impossible to accurately quantify the correlation of features between adjacent pixels. Thus with Upwards Correlation, we can get both false positives and negatives in flame detection. The new invention looks primarily for extremes in negative correlation—a situation which is not affected by thermal bleed.